Method for auto-ignition operation and computer storage device for use with an internal combustion engine

ABSTRACT

An internal combustion engine that can be operated in compression ignition mode, comprising a fuel injector ( 2 ) for each cylinder; a fuel injection control unit ( 4 ) for controlling fuel injection quantity and a piston ( 5 ) in each cylinder whose compression action causes a mixture of air and fuel to be ignited. The engine is further provided with inlet and outlet valves ( 6, 7 ) and sensors ( 12 - 16 ) for measuring various engine operating parameters, is disclosed. During compression ignition mode, the control unit controls the fuel injector to perform a first fuel injection before, and a second fuel injection after top dead center of the piston stroke during or after a negative valve overlap period. A method for operating the engine and a computer readable storage device ( 4 ) having stored therein data representing instructions executable by a computer to implement a compression ignition for an internal combustion engine is also described.

The present application is a continuation of U.S. patent applicationSer. No. 11/610,445 filed on Dec. 13, 2006, which is a divisional ofU.S. patent application Ser. No. 11/131,756, filed on May 17, 2005, nowU.S. Pat. No. 7,156,070 issued Jan. 2, 2007, which is a divisional ofU.S. patent application Ser. No. 10/747,023, filed on Dec. 23, 2003, nowU.S. Pat. No. 6,910,449 issued Jun. 28, 2005, which claims priority toEuropean Patent Application No. 02029060.7, filed Dec. 30, 2002, titledINTERNAL COMBUSTION ENGINE, METHOD FOR AUTO-IGNITION OPERATION ANDCOMPUTER READABLE STORAGE DEVICE, naming Hans Ström and Lucien Koopmansas inventors, and claims priority to European Patent Application No.02029091.2, filed Dec. 30, 2002 titled, INTERNAL COMBUSTION ENGINE,METHOD FOR AUTO-IGNITION OPERATION AND COMPUTER READABLE STORAGE DEVICE,naming Hans Ström and Lucien Koopmans as inventors, the entire contentsof which are incorporated herein by reference.

BACKGROUND AND TECHNICAL FIELD

The invention relates to an internal combustion engine that can beoperated in a homogeneous charge compression ignition combustion mode,as well as a method for controlling such an engine.

SUMMARY

To improve thermal efficiency of gasoline internal combustion engines,lean burn is known to give enhanced thermal efficiency by reducingpumping losses and increasing ratio of specific heats. Generallyspeaking, lean burn is known to give low fuel consumption and low NOxemissions. There is, however, a limit at which an engine can be operatedwith a lean air/fuel mixture because of misfire and combustioninstability as a result of a slow burn. Known methods to extend the leanlimit include improving ignitability of the mixture by enhancing thefuel preparation, for example using atomised fuel or vaporised fuel, andincreasing the flame speed by introducing charge motion and turbulencein the air/fuel mixture. Finally, combustion by auto-ignition, orhomogeneous charge compression ignition, has been proposed for operatingan engine with very lean or diluted air/fuel mixtures.

When certain conditions are met within a homogeneous charge of leanair/fuel mixture during low load operation, homogeneous chargecompression ignition can occur wherein bulk combustion takes placeinitiated simultaneously from many ignition sites within the charge,resulting in very stable power output, very clean combustion and highfuel conversion efficiency. NOx emission produced in controlledhomogeneous charge compression ignition combustion is extremely low incomparison with spark ignition combustion based on propagating flamefront and heterogeneous charge compression ignition combustion based onan attached diffusion flame. In the latter two cases represented byspark ignition engine and diesel engine, respectively, the burnt gastemperature is highly heterogeneous within the charge with very highlocal temperature values creating high NOx emission. By contrast, incontrolled homogeneous charge compression ignition combustion where thecombustion is uniformly distributed throughout the charge from manyignition sites, the burnt gas temperature is substantially homogeneouswith much lower local temperature values resulting in very low NOxemission.

Engines operating under controlled homogeneous charge compressionignition combustion have already been successfully demonstrated intwo-stroke gasoline engines using a conventional compression ratio. Itis believed that the high proportion of burnt gases remaining from theprevious cycle, i.e., the residual content, within the two-stroke enginecombustion chamber is responsible for providing the hot chargetemperature and active fuel radicals necessary to promote homogeneouscharge compression ignition in a very lean air/fuel mixture. Infour-stroke engines, because the residual content is low, homogeneouscharge compression ignition is more difficult to achieve, but can beinduced by heating the intake air to a high temperature or bysignificantly increasing the compression ratio. This effect can also beachieved by retaining a part of the hot exhaust gas, or residuals, bycontrolling the timing of the intake and exhaust valves.

In all the above cases, the range of engine speeds and loads in whichcontrolled homogeneous charge compression ignition combustion can beachieved is relatively narrow. The fuel used also has a significanteffect on the operating range; for example, diesel and methanol fuelshave wider auto-ignition ranges than gasoline fuel. A further problem isto achieve ignition at a particular time with maintained combustionstability, while avoiding engine knocking and misfiring.

Hence an may be desirable, in some cases, to provide a means forcontrolling the ignition timing during auto-ignition, which means allowsfor monitoring of current combustions and for correction of subsequentcombustions dependent on the outcome of the monitoring process.

The above problems can be solved, in some cases, by an arrangement, amethod and a computer readable storage device for controllinghomogeneous charge compression ignition combustion, as described in moredetail below.

One example embodiment relates to an internal combustion enginepreferably, but not necessarily, provided with variable valve timing(VVT), cam profile switching (CPS), direct fuel injection (DI), andmeans for boosting the manifold absolute pressure (turbocharger,compressor etc.). The following text will be mainly concentrated onembodiments including the above features. However, the general principleof the invention as claimed is also applicable to, for instance,stationary aspirating engines with fixed valve timing and a standardcamshaft. Such engines are often operated at fixed speeds and loads andare not subject to the transients normally occurring in, for instance,engines for vehicles. Hence a stationary engine can be operatedcontinuously in HCCI-mode.

Also, although the following examples relate to gasoline fuels, anengine operating according to principles of the invention can be adaptedto use most commonly available fuels, such as diesel, kerosene, naturalgas, and others.

The engine is possible to be operated on homogeneous charge compressionignition (HCCI) combustion mode. This is a combustion mode, differentthan conventional spark ignited (SI) combustion mode, in order to reducefuel consumption in combination with ultra low NOx emissions. In thismode, a mixture containing fuel, air and combustion residuals iscompressed with a compression ratio between 10.5 and 12 to autoignition. The HCCI combustion has no moving flame front, incontradiction to a SI combustion that has a moving flame front. The lackof a flame front reduces temperature and increases the heat release ratehence increases the thermal efficiency of the combustion. The combustionresiduals are captured when operating the engine with a negative valveoverlap. Residuals increase the temperature of the mixture so that theauto ignition temperature is reached before piston top dead center (TDC)and dilute the mixture so that the heat release rate decreases to anacceptable level. By controlling the heat release, cycle-to-cyclevariations (COV), noise and knocking combustion can be reduced. Thenegative valve overlap is achieved when the exhaust valve is closedbefore piston TDC and the inlet valve is opened after piston TDC in thegas exchange phase of the combustion.

The acquired valve timing for the negative overlap can be achieved byusing VVT and CPS, hence switching from conventional SI valve timing toHCCI valve timing with a shorter the valve opening duration and/or valvelift.

One embodiment of the invention uses a gasoline internal combustionengine provided with at least one cylinder and arranged to be switchedbetween spark ignition mode and compression ignition mode. The enginecomprises a fuel injector, through which gasoline fuel is injected intoa combustion chamber, for each cylinder and a fuel injection controlunit that controls gasoline fuel injection quantity per combustion cycleinjected through each fuel injector. Fuel injection is achieved by meansof direct injection (DI) into each combustion chamber.

A spark may be sustained in HCCI mode in order to keep the spark plugfrom fouling and, although the gas mixture is arranged to self ignite,contribute to an increased combustion stability and avoidance ofmisfire.

A reciprocating piston is arranged in each engine cylinder whosecompression action causes a mixture of air and gasoline fuel within thecombustion chamber to be ignited. Gas exchange is controlled by at leastone inlet valve preferably, but not necessarily, provided with variablevalve timing per cylinder for admitting a combustible gas, such as air,and at least one exhaust valve preferably, but not necessarily, providedwith variable valve timing per cylinder for exhausting combusted gases.

The combustion process is monitored by sensors for measuring engineknocking and combustion stability. The knock sensor can be of thepiezo-electric type, which may also be used for continuous monitoring ofcylinder pressure. The combustion stability sensor may be anacceleration type sensor, such as a flywheel sensor, or an ion currentsensor. Alternatively, both said sensors can be replaced by a singlein-cylinder piezoelectric pressure sensor. By processing the output fromsuch a sensor, it is possible to obtain a signal representing engineknock and a signal representing engine stability.

According to one embodiment, the engine is arranged to switch betweenSI-mode to HCCI-mode when certain operating parameters are fulfilled.During compression ignition mode, the exhaust valve is arranged to closebefore top dead center during an exhaust stroke of the piston and theintake valve is opened after top dead center during an induction strokeof the piston. This creates a period of negative valve overlap, duringwhich exhaust and intake valves are closed. The fuel injection controlunit is arranged to control the fuel injection quantity so as to performa first fuel injection before top dead center of the piston exhauststroke and to perform at least a further fuel injection after top deadcenter of the piston stroke during or after the interval of the gasexchange phase when both of the exhaust and intake valves are closed.

In the following text the first injection will generally be referred toas a pilot injection, while any subsequent injection or injections willbe referred to as a main injection. The pilot or first fuel injectionoccurs in the interval between closure of the exhaust valve and top deadcenter of the piston exhaust stroke. At least one further fuel injectionoccurs in the interval after top dead center of the piston exhauststroke and before top dead center of a subsequent piston compressionstroke. Injection near the top dead center is generally avoided to avoidformation of soot in the combustion chamber. Said further, or maininjection may be a single second injection or comprise two or moreinjections. The total amount of the main injection can exceed the amountinjected in the pilot injection (although in some cases it always doesso).

According to a further embodiment of the invention, the amount of fuelinjected during first injection and one or more second, main injectionsis determined by the fuel injection control unit on the basis ofcomparison between predetermined limit values for a knock signaltransmitted from said engine knocking sensor and a stability signaltransmitted from said combustion stability sensor respectively.

According to a further embodiment of the invention, the total amount offuel injected during the first and second injections is substantiallyconstant under constant engine operating conditions.

According to a further embodiment of the invention the distribution ofinjected fuel between the first and the second injection is adjusted inincrements based on the comparison made by the injection control unit.The adjustments are determined by the following conditions;

-   -   if the knock signal and the stability signal are below their        predetermined limit values, the fuel injection control unit is        arranged to reduce the amount injected during the first        injection;    -   if the knock signal and the stability signal are above their        predetermined limit values, the fuel injection control unit is        arranged to increase the amount injected during the first        injection;    -   if the knock signal is above its predetermined limit value and        the stability signal is below its predetermined limit value, the        fuel injection control unit is arranged to reduce the amount        injected during the first injection; or    -   if the knock signal is below a predetermined limit value and the        stability signal is above a predetermined limit value, the fuel        injection control unit is arranged to increase the amount        injected during the first injection.

In the latter case, where it is detected that the knock signal is belowa predetermined limit value and the stability signal is above apredetermined limit value, two further conditions are applied;

-   -   if it is detected that a combustion peak pressure occurs earlier        than a predetermined point in time, the fuel injection control        unit is arranged to increase the amount of fuel injected during        the first injection for the subsequent combustion cycle; or    -   if it is detected that a combustion peak pressure occurs later        than a predetermined point in time, the fuel injection control        unit is arranged to increase the amount of fuel injected during        the first injection for the cycle following the subsequent        combustion cycle.

Although the relative amount of fuel injected is adjustable, the firstinjection is arranged as a pilot injection, while the second injectionis a main injection. Hence, the quantity of the first injection ispreferably selected to be greater than zero but less than 45% of thetotal amount of injected fuel, comprising the pilot and the maininjection. The fuel of the pilot injection will react in the residualexhaust gas, forming radicals, intermediates or combustion products.This reaction can be exothermic hence heating the residuals, resultingin earlier timing of the auto ignition temperature. As will be describedlater, this reaction takes place in the presence of excess oxygen, whichmay be present in the combustion chamber following the previouscombustion cycle. The retained residual exhaust gas will have the sameλ-value as the air/fuel mixture of the previous combustion cycle.

According to a further embodiment, the invention also relates to amethod for operating a internal combustion engine provided with at leastone cylinder and arranged to be switched between spark ignition mode andcompression ignition mode, or to be operated continuously in compressionignition mode. As described above, the engine comprises a fuel injector,through which gasoline fuel is injected into a combustion chamber, foreach cylinder, a fuel injection control unit that controls gasoline fuelinjection quantity per combustion cycle injected through each fuelinjector, and a piston in the engine cylinder whose compression actioncauses a mixture of air and gasoline fuel within the combustion chamberto be ignited during compression ignition mode. The gas exchange iscontrolled by at least one inlet valve preferably, but not necessarily,provided with variable valve timing per cylinder for admittingcombustible gas and at least one exhaust valve with variable valvetiming per cylinder for exhausting combusted gases. The combustionprocess may be monitored by a sensor for measuring engine knocking andgenerating a knock signal and a sensor for measuring combustionstability and generating a stability signal.

The method for controlling the engine involves adjusting opening andclosing timings of the inlet valve and the exhaust valve so that thepiston moving within the cylinder performs an intake phase, acompression phase, an expansion phase, an exhaust phase and an exhaustretaining phase, while performing a split fuel injection during andafter the exhaust retaining phase.

According to a further embodiment of the invention, the method includesthe following steps during the exhaust retaining phase:

-   -   closing the exhaust valve before top dead center during an        exhaust stroke of the piston and opening the intake valve after        top dead center during an induction stroke of the piston; and    -   controlling the fuel injection control unit so as to perform a        first fuel injection before top dead center of the piston stroke        and to perform a second gasoline fuel injection in the interval        after top dead center of the piston exhaust stroke and before        top dead center of a subsequent piston compression stroke.

According to a further embodiment of the invention, the method alsoinvolves comparing each of the knock signal and the stability signalwith a respective predetermined limit value. Depending on the outcome ofsaid comparison, the quantities of the first and second fuel injectionsare adjusted by;

-   -   reducing the amount injected during the first injection if both        signals are below their predetermined limit values;    -   increasing the amount injected during the first injection if        both signals are above their predetermined limit values;    -   reducing the amount injected during the first injection if the        knock signal is above its predetermined limit value and the        stability signal is below its predetermined limit value; or    -   increasing the amount injected during the first injection if the        knock signal is below a predetermined limit value and the        stability signal is above a predetermined limit value.

For the latter case, where the knock signal is below a predeterminedlimit value and the stability signal is above a predetermined limitvalue, a further condition is applied. A combustion peak pressure timingis detected to allow a comparison of the actual timing of the peakpressure with a predicted timing of peak pressure. The peak pressuretiming may be detected by a suitable existing sensor or by a separatepressure sensor. The quantities of the first and second fuel injectionsare adjusted by;

-   -   increasing the amount of fuel injected during the first        injection for the subsequent combustion cycle if the actual        timing occurs earlier than said predicted timing; or    -   increasing the amount of fuel injected during the first        injection for the cycle following the subsequent combustion        cycle if the actual timing occurs later than said predicted        timing.

According to a further embodiment, the invention also relates to acomputer readable storage device having stored therein data representinginstructions executable by a computer to implement a compressionignition for a gasoline internal combustion engine, the engine having apiston disposed in a cylinder to define a combustion chamber, intakevalves for admitting fresh air into the cylinder, a fuel injector forinjecting fuel into the combustion chamber, and exhaust valves fordischarging exhaust gas resulting from combustion within the cylinder,wherein opening and closing timings of the intake means and opening andclosing timings of the exhaust valves are adjustable. The computerreadable storage device comprises:

-   -   instructions for adjusting opening and closing timings of the        intake means and opening and closing timings of the exhaust        means such that the piston reciprocates within the cylinder to        perform an exhaust phase, an exhaust gas retaining phase, an        intake phase, a compression phase, and an expansion phase;    -   instructions for providing a first start time of a first fuel        injection by the fuel injector during said exhaust gas retaining        phase and a further start time of a second fuel injection by the        fuel injector in the interval after top dead center of the        piston exhaust stroke and before top dead center of a subsequent        piston compression stroke;    -   instructions for determining a portion of total fuel quantity        and the remainder of said total fuel quantity;    -   instructions for determining a first fuel injection control        signal indicative of said portion of said total fuel quantity        and applying said first fuel injection control signal to the        fuel injector at said first start time to control fuel quantity        injected for said first fuel injection; and    -   instructions for determining a further fuel injection control        signal indicative of the remainder of said total fuel quantity        and applying said further fuel injection control signal to the        fuel injector at said further start time to control fuel        quantity injected for said second fuel injection.

According to a further embodiment the invention, the computer readablestorage device also comprises instructions for determining the fuelinjection control signals indicative of the amount of fuel injectedduring the first and second injection, as determined by the fuelinjection control unit on the basis of comparison between predeterminedlimit values for a knock signal transmitted from an engine knockingsensor and stability signals transmitted from a combustion stabilitysensor respectively. The control unit estimates the cycle-to cyclevariations (COV) by storing and evaluating a number of subsequentstability signals.

The computer readable storage device comprises:

-   -   instructions for adjusting opening and closing timings of the        intake valves and opening and closing timings of the exhaust        valves such that the piston reciprocates within the cylinder to        perform an exhaust phase, an exhaust gas retaining phase, an        intake phase, a compression phase, and an expansion phase;    -   instructions for providing a first start time of a first fuel        injection by the fuel injector during said exhaust gas retaining        phase and a further start time of at least one further fuel        injection by the fuel injector in the interval after top dead        center of the piston stroke and before top dead center of a        subsequent piston compression stroke;    -   instructions for determining a portion of total fuel quantity        and the remainder of said total fuel quantity;    -   instructions for determining a first fuel injection control        signal indicative of said portion of said total fuel quantity        and applying said first fuel injection control signal to the        fuel injector at said first start time to control fuel quantity        injected for said first fuel injection; and    -   instructions for determining a further fuel injection control        signal indicative of the remainder of said total fuel quantity        and applying said further fuel injection control signal to the        fuel injector at said further start time to control fuel        quantity injected for said further fuel injection.

In a further embodiment, the computer readable storage device comprises:

-   -   instructions for determining the fuel injection control signals        indicative of the amount of fuel injected during the first        injection and a second injection, as determined by the fuel        injection control unit on the basis of comparison between        predetermined limit values for a knock signal transmitted from        an engine knocking sensor and a stability signal transmitted        from a combustion stability sensor respectively.

BRIEF DESCRIPTION OF DRAWINGS

In the following text, the invention will be described in detail withreference to the attached drawings. These drawings are used forillustration only and do not in any way limit the scope of theinvention. In the drawings:

FIG. 1 shows a schematic internal combustion engine according to theinvention;

FIG. 2 shows a diagram illustrating the variation of cylinder pressureover crank angle for HCCI- and SI-mode;

FIG. 3 shows a diagram illustrating the cylinder pressure over time fora number of sequential combustion cycles in HCCI-mode;

FIG. 4 shows a diagram for a control loop for combustion, stability andknock control in HCCI-mode.

DETAILED DESCRIPTION

FIG. 1 shows a schematic illustration of an internal combustion engineaccording to the invention. The engine is provided with at least onecylinder 1 and comprises a fuel injector 2, through which fuel isinjected into a combustion chamber 3, for each cylinder. A fuelinjection control unit 4 controls fuel injection quantity per combustioncycle injected through each fuel injector. A piston 5 in the enginecylinder has a compression action that causes a mixture of air and fuelwithin the combustion chamber to be ignited during HCCI-mode. Thecylinder is provided with at least one inlet valve 6 for admitting gaswhich includes fresh air into said cylinder and at least one exhaustvalve 7 for exhausting combusted gases from said cylinder. Air issupplied through an intake conduit 9 connected to an intake manifold,while exhaust gas is exhausted through an exhaust conduit 10. DuringSI-mode, the ignition of the fuel/air mixture is ignited by a spark plug8.

The control unit receives signals from at least one sensor for measuringengine operation parameters, which sensors include a combustion chamberpressure sensor 11, an intake manifold pressure sensor 12 and a λ-probe13 in the exhaust conduit, as well as temperature sensors for intake air14, engine coolant 15 and engine oil 16. The control unit controls theintake and exhaust valves 6, 7 by means of valve actuators 17, 18. Theactuators may be either electrically or mechanically operated.

FIG. 2 shows a diagram illustrating the variation of cylinder pressureover crank angle for HCCI- and SI-mode. As can be seen from the curvesin the diagram, the engine can be operated in homogeneous chargecompression ignition (HCCI) combustion mode and in conventional sparkignited (SI) combustion mode. The HCCI combustion has no moving flamefront, as opposed to a SI combustion that has a moving flame front. Thelack of a flame front reduces temperature and increases the heat releaserate hence increases the thermal efficiency of the combustion. This willresult in a considerably higher peak pressure after ignition (IG);typically in excess of 40 bar, as opposed to about 20 bar in SI mode.The main difference between the HCCI- and SI modes is that a part of thecombustion residuals are captured by operating the engine with anegative valve overlap. The negative valve overlap is achieved byclosing the exhaust valve, or EV, before piston TDC (EVC) and openingthe inlet valve, or IV, after piston TDC (IVO) in the gas exchange (GE)phase of the combustion, as illustrated in FIG. 2. During the air intakephase, residuals increase the temperature of the mixture so that theauto ignition temperature is reached before piston top dead center (TDC)and dilutes the mixture so that the heat release rate decreases to anacceptable level. By controlling the heat release, noise and knockingcombustion can be reduced.

A split fuel injection is used having a pilot direct fuel injection (PI)before TDC during the negative valve overlap and a main direct fuelinjection (MI) after TDC of the negative valve overlap. The relativequantities of fuel injected during the pilot and the main injections canbe varied and are calculated and controlled by a fuel injection controlunit (not shown). The fuel of the pilot injection (PI) will react in theretained residuals, forming radicals, intermediates or combustionproducts. This reaction can be exothermic hence heating the residuals,resulting in earlier timing of the auto ignition temperature. Aprerequisite for this reaction is the presence of excess oxygen, withoutwhich the reaction will stop before it is completed. When the engine isoperated in HCCI-mode the control unit must adjust the value of λ to besufficiently high for all engine operating conditions to ensure this.The total quantity of injected fuel for the pilot and the main injectionis substantially constant with respect to the current engine operatingconditions, such as engine speed, engine load and efficiency. Thequantity of the first injection is preferably selected to be in therange of 0<PI<45% of the total amount of injected fuel.

Due to the demand for dilution, which controls the rate of heat release,only the part load regime of the engine is used for HCCI combustionmode. The auto ignition timing for HCCI operation can be controlled bythe pilot fuel injection and/or the captured amount of residuals and/orthe absolute manifold pressure. The latter may be controlled byincreasing the pressure of the intake air by means of a compressor orturbocharger.

When operating the engine, engine knocking, low combustion stability anda high noise level has to be avoided. Knocking, which is also a sourceof noise, is detected by measuring the peak pressure and/or pressurevariations caused by a too rapid heat release during the expansionphase. Knocking occurs when the peak pressure exceeds an expectedmaximum pressure, or when a series of rapid pressure variations occurduring the expansion phase. Low combustion stability is indicated byhigh cycle to cycle variations of the pressure during combustion.Typically, an engine operated in HCCI mode may oscillate between a latephased combustion (low cylinder pressure) and a subsequent early phasedcombustion (high cylinder pressure). When the engine is operating in theHCCI-mode, at least four combinations of sequential combustion cyclesare possible. This is illustrated in FIGS. 3A-D.

A more detailed explanation of the cycle-to-cycle variation (COV) andhow this may oscillate under different engine operating conditions canbe found in the SAE-paper SAE 2002-01-0110, the entirety of which ishereby incorporated into the description by reference. The SAE-paperdiscusses the cycle-to-cycle variations (COV) during HCCI-operation. Theoscillating nature of COV and the effect of exhaust valve closure timingon combustion stability are described.

In all cases shown in FIGS. 2A-D a control unit (not shown) evaluatesthe signals from sensors that indicate knock and combustion stability.In the figures, a knock signal is deemed to be high if the peak pressureduring combustion exceeds an expected pressure level, indicated by ahorizontal line in all FIGS. 3A-D. When a COV signal is deemed to behigh, this is indicated by a reduced peak pressure during combustion.Due to the cyclic nature of the COV signal, the reduction in peakpressure generally occurs every second combustion cycle. All figuresindicate the timing of the piston top dead center (TDC) and the exhaustvalve closure (EVC).

FIG. 3A shows the cylinder pressure for a case where the knock signal islow and the oscillating COV signal is low. In this case the noise levelcan be unacceptable. According to this embodiment, the combustionphasing is retarded by decreasing the amount of fuel injected in thepilot injection, in combination with an increase of the amount for themain fuel injection in order to keep load and lambda constant. Noiselevel will be reduced with a later phased, or retarded, combustion.

FIG. 3B shows the cylinder pressure for a case where the knock signal ishigh and the COV signal is high. This indicates high knocking cycleswith early phased combustion cycles, alternating with late phasedcombustion cycles. When both the knock signal and the COV signal is highthe amount of fuel injected in the pilot injection is increased, whilethe amount for the main fuel injection is decreased in order to keepload and lambda constant. The combustion is then phased earlier, oradvanced, in the next cycle and engine knock is decreased. For aconventional control strategy, detection of knocking would cause thecombustion phasing to be retarded. In this mode of operation such astrategy would cause misfire.

FIG. 3C shows the cylinder pressure for a case where the knock signal ishigh and the COV signal is low. In this case the amount of fuel injectedin the pilot injection is decreased, while the amount for the main fuelinjection is increased in order to keep load and lambda constant. Thecombustion is then phased later in the next cycle and engine knock isdecreased. In this context, the term “next cycle” refers to the cyclefollowing immediately after the current cycle.

FIGS. 3B and 3D show the cylinder pressure for a case where the knocksignal is low and the COV signal is high. This indicates low knockingcycles with early phased combustion cycles, alternating with late phasedcombustion cycles. For a conventional control strategy, this wouldresult in an immediate advance of the combustion phasing to avoidproblems with stability. However, if this adjustment occurs immediatelyafter a late phased cycle, the result would most likely be engineknocking in the next cycle.

According to the invention, the control unit evaluates the signals fromsensors that indicate knock, combustion stability and combustionphasing. The latter is preferably achieved by detecting the location ofpeak pressure (LPP). When the knock signal is low, the COV signal ishigh and LPP is early, the amount of fuel injected in the pilotinjection is increased, while the amount for the main fuel injection isdecreased in order to keep load and lambda constant. This will phase thecombustion of the next cycle earlier than it would have been withoutinjection adjustment and combustion stability is increased.

However, if LPP is sensed late, the amount of fuel injected in the pilotinjection is increased, while the amount for the main fuel injection isdecreased for the cycle after the next cycle. This delay avoids an evenearlier and perhaps knocking combustion for the next cycle. If, for somereason, the time taken by the control unit to perform the necessarycalculations exceeds the start of the next, immediately following EVCevent, then the adjustment of the injections is skipped for two cycles.This is indicated in FIGS. 3B and 3D, where a first event EVC₁ isassumed to be missed. The control unit will then skip the cyclesincluding EVC₁ and the following event EVC₂, to execute the adjustedinjection after the start of a third event EVC₃.

FIG. 4 shows a schematic diagram for a control strategy for managing thecombustion control, engine knocking and combustion stability, usingvariations of the pilot fuel injection which is possible to alter fromcycle to cycle. The control strategy involves reading values for pilotand main injection from a map stored in the control unit. Based on thesevalues the control unit performs an evaluation of the output signalsfrom multiple sensors, such as a knocking sensor, a combustion stabilitysensor and a pressure sensor, and calculates required corrections of theamount of fuel injected in the pilot and main injections accordingly.The corrections are generally very small from cycle to cycle and themagnitude of the incremental steps is controlled by and dependent on theaccuracy of the PID regulator used. However, for reasons of clarity,FIG. 4 describes combustion control for steady state condition in orderto illustrate the general principle of the invention. In actual use thecontrol unit applies a dynamic regulation dependent on current engineoperating conditions.

When the engine is switched from SI-mode to HCCI-mode, the control loopis initiated by the injection control unit. After transmitting a commandto start S1 the control loop, the control unit reads the output signalstransmitted from a number of sensors S2. In this embodiment the sensorsused are a knocking sensor, a combustion stability sensor and a pressuresensor. The control unit will then compare the knock signal with apredetermined limit value S3 to determine whether the signal is high orlow. If the knock signal is high the control unit will compare thestability signal, also referred to as COV, with a further predeterminedlimit value S4. If the COV signal is also high, then the control unitwill immediately increase the amount of fuel injected during the pilotinjection S5, that is, for the next cycle. As described above, theamount of fuel injected during the main injection will be decreasedaccordingly. If, on the other hand, the COV signal is low, then thecontrol unit will immediately decrease the amount of fuel injectedduring the pilot injection S6, that is, for the next cycle.

Should the control unit determine that the knock signal is lower thanthe predetermined limit value S3, the control unit will compare the COVsignal to a predetermined limit value S7, identical to that of step S4.If the COV signal is low, then the control unit will immediatelydecrease the amount of fuel injected during the pilot injection S6, thatis, for the next cycle.

However, if it is determined that the knock signal is low S3 and thatthe COV signal is high, a further comparison is made using a signalindicating cylinder pressure plotted over time. The control unit canthen determine the location of peak pressure (LPP), that is, when themaximum pressure occurs during combustion. The control unit can thendetermine if the LPP has occurred early or late S8 with respect to anestimated or desired point in time. If the LPP has occurred early, thenthe control unit will immediately increase the amount of fuel injectedduring the pilot injection S9, that is, for the next cycle immediatelythe current cycle. If, on the other hand, the LPP has occurred late,then the control unit will increase the amount of fuel injected duringthe pilot injection S10 for the subsequent cycle, that is the cyclefollowing the next cycle. This delay avoids an even earlier and perhapsknocking combustion for the next cycle, as described above, andcounteracts possible oscillations caused by cycle-to-cycle variations.

For all the operating conditions described above, the control loop iscarried out continuously for each combustion cycle until the controlunit determines the HCCI operation is no longer possible S11. Thecontrol unit will then end the procedure S12 and switch to SI-mode.

The invention is not limited to the embodiments described above and maybe varied freely within the scope of the appended claims.

1. A method for controlling an engine having a variable valve actuator, the method comprising: operating the engine in a first mode where the engine carries out spark ignition combustion of air and fuel; and changing operation to a second mode where the engine carries out homogenous charge compression ignition combustion of air and fuel, where said variable valve actuator unit is adjusted in response to said change to provide a shorter valve timing.
 2. The method of claim 1 wherein said variable valve actuator is a variable cam timing actuator.
 3. The method of claim 1 wherein said variable valve actuator is a variable valve lift actuator.
 4. The method of claim 3 wherein said variable valve actuator is a cam profile switching actuator.
 5. The method of claim 1 wherein said variable valve actuator is a cam profile switching actuator.
 6. The method of claim 1 wherein spark ignition is utilized at least during a portion of said operation in said second mode. 